Oxygen binding in blood of Xenopus laevis (Amphibia) and evidence against Root effect

Magnitude of the Root effect in red blood cells and haemoglobin solutions of fishes: a tribute to August Krogh

AIM

The ability of high carbon dioxide tensions or low pH to reduce blood oxygen binding even at high oxygen tensions, first observed by August Krogh and Isabella Leitch in 1919 and now known as the Root effect, was studied in red blood cells and haemoglobin solutions of several fish species.

METHODS

Red blood cells in physiological saline were acidified at atmospheric oxygen tension by increasing carbon dioxide tensions and the percentage decrease in oxygen content was used to quantify the Root effect. Haemoglobin was incubated in air-equilibrated citrate buffers between pH 5 and 7 and the percentage decrease in oxygen saturation relative to pH 8 determined by spectral deconvolution.

RESULTS

The maximal magnitude of the Root effect in citrate-buffered haemoglobin solutions closely matched the value in blood or red blood cells of 11 vertebrates over a Root effect range between 3 and 80%. Contrary to previous reports, there was no evidence for a significant Root effect in red blood cells or haemoglobin solutions of the wels catfish, but a significant Root effect under both conditions in the Siberian sturgeon.

CONCLUSIONS

Under the conditions employed in this study, the maximal Root effect of citrate-buffered haemoglobin solutions closely resembles the maximal Root effect in red blood cells. This strengthens previous studies on the evolution of the Root effect and its role in oxygen concentration at the retina and swimbladder of a large number of fishes that were based on Root effect measurements in haemoglobin solutions.

Haemoglobin function and respiratory status of the Port Jackson shark, Heterodontus portusjacksoni, in response to lowered salinity

Haemoglobin function and respiratory status of sub-adult sharks, Heterodontus portusjacksoni was investigated for up to 1 week following transfer from 100% to either 75% or 50% seawater. Metabolic rates were unusually low and arterial-venous differences in blood O2 small. Haemodilution from osmotic inflow lowered haematocrit and reduced blood O2 content by up to 50%. There was no change in O2 consumption rate, blood O2 partial pressure, cardiac output, or the arterial-venous O2 content difference, and thus O2 delivery was maintained. Ventilation was acutely elevated but returned to normal within 24 h. The O2 delivery to the tissues was facilitated by decreased blood O2-affinity that could not be simply ascribed to changes in the osmolyte concentration. The Hb was unaffected by changes in intra-erythrocyte fluid urea or trimethylamine-N-oxide (TMAO) but was sensitive to changes in NaCl. The Bohr shifts in whole blood were low and there was little role for pH in modulating O2 transport. Venous Hb saturation remained close to 65%, at the steepest part of the in vivo O2 equilibrium curve, such that O2 unloading could be facilitated by small reductions in pressure without increasing cardiac or ventilatory work. H. portusjacksoni tolerated 50% seawater for at least 1 month, but there was little evidence of respiratory responses being adaptive which instead appeared to be consequential on changes in osmotic and ionic status.

Interactions between ion and gas transfer in freshwater teleost fish

Carbonic anhydrase and proton ATPase are co-distributed, being restricted to the apical regions of the gill epithelium of freshwater teleosts. Carbonic anhydrase supplies protons to the apical proton ATPase. Carbonic anhydrase is absent from the basal regions of the gill epithelium. Plasma flowing through the gills has no available carbonic anhydrase activity and plasma CO2/bicarbonate reactions are uncatalyzed. Thus, bicarbonate dehydration in plasma is negligible, and catalyzed bicarbonate dehydration occurs in erythrocytes in blood flowing through the gills. This results in tight coupling of carbon dioxide excretion to oxygen uptake and the evolution of hemoglobins with large Haldane effects but low buffering capacities, typical of many freshwater teleosts. Tight coupling of carbon dioxide and oxygen transfer in these fish also ensures that the Root shift does not impair oxygen uptake at the gills. Under these conditions, there is a selective advantage for hemoglobins with a Root shift. The presence of a Root shift augments oxygen transfer to the tissues in general and the eye and swimbladder in particular.

Cooperativity and allosteric regulation in non-mammalian vertebrate haemoglobins

1. This review illustrates the vast range of molecular functions expressed in non-mammalian vertebrate haemoglobins; with particular reference to the degree of aggregation of haemoglobin subunits and their interactions with allosteric effectors. 2. In at least the broadest sense, these properties suggest that haemoglobin function in non-mammalian vertebrates can be viewed against the evolutionary hierarchy of organisms rather than from a purely adaptive perspective.

CO2 transport in agnathan blood: evidence of erythrocyte Cl-/HCO3- exchange limitations

CO2 transport properties of blood were examined in the lamprey Petromyzon marinus and the hagfish Myxine glutinosa. In order to evaluate possible chloride/bicarbonate exchange limitations, experiments were conducted under control conditions and in the presence of an ionophore to permit equilibrium distribution of chloride, bicarbonate, and protons across the erythrocyte membrane. The ionophore, tri-n-propyl tin chloride, markedly altered the CO2 transport properties and apparent nonbicarbonate buffering characteristics of the blood of Petromyzon marinus. In addition, the distributions of protons, bicarbonate and chloride ions across the erythrocyte membrane were very different from each other under control conditions, but became very similar in the presence of the anionic ionophore. The CO2 transport properties of the blood of Myxine glutinosa were not significantly different in the presence of the ionophore. Small but significant changes were observed, however, in erythrocyte pH, chloride concentration and water content in the presence of tri-n-propyl tin chloride. These results demonstrate that chloride/bicarbonate exchange limitations and possibly active transport of protons contribute to the unique CO2 transport properties in the blood of the lamprey, Petromyzon marinus. In the hagfish, Myxine glutinosa, the importance of anion exchange limitations or active proton transport with regard to the CO2 carrying properties of the blood are clearly much less than in the lamprey under the in vitro conditions of this study.

Analysis of oxygen binding by Xenopus laevis hemoglobin: implications for the Root effect

We have measured oxygen binding properties for red cell suspensions and stripped hemolysate of Xenopus laevis (XL) in order to answer the question whether XL hemoglobin exhibits a Root effect. The present results show that under physiological conditions XL red cells do not exhibit all the criteria for a Root effect. Compared to human Hb A, stripped XL hemoglobin has a low oxygen affinity, a normal alkaline Bohr effect and a lower interaction with 2,3-diphosphoglycerate, its physiological allosteric effector in red cells. The values for the Hill coefficients are lower than those for human hemoglobin but XL Hb remains cooperative (n50 approximately 2), even at pH values below 6. Attempts to mimick some of the criteria for the Root effect were carried out in pure Hb A solution upon addition of potent allosteric effectors. This leads to low cooperativity and less than 100% oxygen saturation under room air at acidic pH. Under these conditions, mammalian or XL Hb have a maximum proton release at pH approximately 8 and an increased reverse Bohr effect. By contrast, a Root effect Hb exhibits a maximum proton release at neutral pH with the absence of reverse Bohr effect. Therefore the Root effect in fish Hb and the extreme stabilization of the T-state with effectors in mammalian Hb are not an identical phenomenon. Without crystallographic analyses of fish Hb exhibiting a Root effect, the molecular interpretation of this functional property still remains unexplained.

Is there a Root effect in Xenopus hemoglobin?

The reaction of Xenopus hemoglobin with oxygen and carbon monoxide has been reinvestigated over the pH range 8.5-6.0, in the absence and presence of organic phosphates (2,3-diphosphoglycerate or inositol hexakisphosphate), to establish if the tetramer can be stabilized in a T-quaternary state by protons and polyphosphate; the equilibrium and kinetic data indicate that Xenopus hemoglobin does exhibit a Root effect. These new results are discussed with reference to those reported by Bridges et al. [(1985) Resp. Physiol. 61, 125-136] on Xenopus blood and, more generally, to the molecular definition and the structural basis of the Root effect as an extreme form of the Bohr effect.

The Root effect

Considering the presently available data it is clear that the Root effect represents an exaggerated alkaline Bohr effect which occurs in the absence of a normal acid Bohr effect and is associated with a loss of oxygen binding capacity at low pH. Undoubtedly at the molecular level the presence of a Ser residue at position F9(94) beta in these haemoglobin is of primary importance. No Root effect haemoglobin has yet been identified which lacks this substitution. On the other hand however many haemoglobins are known which possess this Ser residue and at the same time lack a Root effect. Other factors arising from interactions at other sites in the haemoglobin molecule are obviously sufficient to negate the otherwise stabilizing effect of this critical Ser residue. The loss of cooperativity of Root effect systems as the pH is lowered is readily explained as due to stabilization of the low affinity T state to such a degree that the switch to the high affinity R state is suppressed even in the fully liganded molecule. The observation of Hill coefficients of less than unity requires that within the T state chain heterogeneity exists such that the alpha and beta chain haems demonstrate significantly different affinities for ligand. The physiological role of Root effect haemoglobins is demonstrably not inevitably linked to the swim bladder but more probably arose from the need to oxygenate the poorly vascularized retina of many fishes.(ABSTRACT TRUNCATED AT 250 WORDS)